1. Field of the Invention
The invention is concerned with techniques for vibratory measurement of the viscosity of fluids, particularly techniques which are capable of measurement of low viscosity fluids using devices suitable for continuous measurement in a flowing stream, commonly referred to as a process viscometer.
2. Background Information
Knowledge of the viscosity of a fluid is of great practical interest in many technical areas, including the measurement and control of printing inks, fuels, lubricants, paints, and coatings. In many cases, such as fuel, lubricant and ink measurement, there is a need for continuous measurement of low viscosity fluid (perhaps 5 to 20 centipoise), while operating in a demanding, high noise environment.
There have been developed many commercial, scientific, and laboratory techniques for viscosity measurement. In all cases, it is necessary to induce a shearing motion in the fluid, since viscosity is a measure of the resistance of a fluid to shear. In some devices that shearing motion is induced by vibration.
One vibratory technique is to twist or flex a tube, or bar, or probe immersed in or containing the fluid under test. For example, U.S. Pat. No. 4,525,610 describes a tube which contains the fluid which is twisted and flexed. U.S. Pat. Nos. 5,228,331, 5,723,771, 6,112,581 and 6,250,136 describe a tube or shaft with a paddle which is twisted in torsion. U.S. Pat. Nos. 4,026,671 and 5,710,374 describe the use of a rod which is vibrated axially while immersed in a fluid. In each case creation and measurement of torsional force and motion is relatively complicated and the systems are generally not sufficiently accurate at low viscosities.
In U.S. Pat. Nos. 6,269,686 B1 and 6,311,549 B1 there are described micro-machined devices in which a small cantilevered element is vibrated.
U.S. Pat. Nos. 5,253,513 and 5,750,884 describe laboratory devices in which the fluid is contained between two plates. A broadband, quasi-random force is imposed on one plate, normal to the face of the plate. Instruments are then used to measure the force, measure the resulting displacement, then to analyze the two waveforms in the frequency domain to determine viscosity. These laboratory devices are complex and not well suited to continuous process measurement.
I have developed a simple two wire viscometer with good sensitivity to even low viscosity fluids using vibrational techniques. I excite flexural motion of a cantilever beam using a brief impulsive electromagnetic force generated by a coil in magnetic proximity to the beam. A magnet is mounted on the beam which couples into the coil to increase the magnetic coupling to the coil and to create electromotive force in the coil as the beam vibrates. A magnetic circuit is also used to improve the coupling. In order to increase the viscous damping effects of the fluid to enable accurate measurement of low viscosities, the flexible beam is positioned close to a rigid post. The fluid in the gap between the beam and the post is sheared as a result of beam vibration normal to the post. The shearing motion of the fluid results in viscous retardation of the flexible beam and attenuation of the vibration. Detection circuitry measures the rate of attenuation which is shown to be related to absolute viscosity.
Excitation and measurement power dissipation in the coil is minimized enabling measurement of the resistance of the coil as a determinant of the temperature of the fluid.